Salt tectonics in contractional fold belts, the Kuqa foreland basin and thrust belt case (Tarim basin, China)

Abstract

The thesis presented here spins off from the 2-year oil-industry project entitled “Salt Tectonics Modelling at Kuqa Foreland Fold and Thrust Belt, Tarim Oilfield” that was a collaboration between the American-based company China Petroleum Corporation (CNPC USA) and GEOMODELS Research Institute (via Fundació Bosch i Gimpera). This research project stems from the need to understand the structural character and evolution of the Kuqa fold- and-thrust belt (NW Xina) in order to identify exploration targets at the Mesozoic subsalt level. Specifically, the aim of the study was to identify exploration targets at the Paleogene subsalt structural level of the Kuqa foreland basin and adjoining fold and thrust belt, in particular beneath the Qiulitage fold-and-thrust system. In this scenario, the project research was focused on the definition and understanding of the different salt structures, their relationship with the geodynamic context and the different types of related hydrocarbon traps.

To achieve these objectives, it was agreed to carry out six regional cross-sections, three of them balanced and restored as well as eleven balanced cross-sections of the Qiulitage fold-and-thrust system, three of them restored. In addition, regional structural maps were produced showing the salt and subsalt structures as well as their relationships with salt distribution and thickness. This line of research has been complemented with the realization of eight numerical discrete-element models and eight scaled sandbox analogue models of tectonic wedges incorporating variations in the rheology of a weak layer and in the syn- kinematic sedimentary rate.

In order to accomplish these tasks, the company provided around 1500 km of 2D seismic lines and geophysical logs from numerous wells drilled in the area. In addition, two field campaigns were organized between June (15 days) and September 2015 (21 days) where structural data were collected to recognize the surface structure of the Kuqa fold-and-thrust belt. Moreover, several transects were realized during these field campaigns to recognize the surface expression of interpreted structures in the seismic lines.

In this context, this thesis deals only with the results obtained from the structural analysis based on surface and subsurface data and analogue modelling. These results are presented in this manuscript which includes the following chapters.

CHAPTER 1 that provides a summary of the thesis.

CHAPTER 2 which provides a general introduction to the geology and the kinematic evolution of the Tian Shan intraplate range. Then, the chapter focuses in the southern frontal structure of the central Tian Shan Range, the Kuqa fold-and-thrust belt, describing both the stratigraphy and the main features of its structure. Finally, the main objectives of this thesis are presented.

CHAPTER 3 that deals with the structural analysis carried out in the Kuqa fold-and- thrust belt with the field and subsurface data. The chapter shows the input data and the interpretation of both surface and subsurface data. The chapter describes the obtained results of three regional cross-sections as well as the regional structural maps of salt distribution and thickness. In addition, the chapter provides a kinematic evolution of the Kuqa fold-and-thrust belt and discusses the main parameters controlling it.

CHAPTER 4 that describes the data, methodology, procedure and results obtained in the analogue modelling experiments. These experiments where designed to analyze the influence of the rheological properties of two superimposed décollement layers but mainly the one of the syn-tectonic sedimentation. So, after a brief introduction to the purpose of the experimental program and the methodology used, this chapter describes the experimental results separating them according to the applied syn-kinematic sedimentary rate. Then, the obtained results are compared and discussed mainly focusing on: the influence of the mechanical properties of a weak layer; the syn-kinematic sedimentary rate in the geometry and kinematics of brittle-viscous tectonic wedges; as well as the interaction between sub-salt and salt-detached structures.

CHAPTER 5 which compares the experimental results with the structure and kinematic of the Kuqa fold-and-thrust belt defined from our structural field and subsurface analyses that is described in the Chapter 3.

CHAPTER 6 which depicts the main conclusions of this thesis.

CHAPTER 7 formed by the references mentioned throughout the text.

El cinturó de plecs i encavalcaments de Kuqa, situat a l’avantpaís meridional de la serralada del Tian Shan occidental, va ser deformat contractivament durant el Mesozoic superior i el Cenozoic tal com queda registrat a les seqüències sin-tectòniques continentals. A més a més, la seva evolució estructural va ser fortament controlada per la presència de sal sin- orogència (d’edat Eocè-Oligocè) i dels décollements pre-sal. En aquest context, presentem un conjunt de sis talls geològics, tres d’ells restituïts, a través del cinturó de plecs i encavalcaments de Kuqa que proporcionen una nova interpretació de la estructura per sota de les evaporites, en la qual els materials paleozoics i mesozoics estan deformats per un apilament d’encavalcaments involucrant (i) un sistema d’encavalcaments de pell fina desenganxats en nivells de carbó triàsics-juràssics, i (ii) un conjunt d’encavalcaments de basament de vergència nord. Les restitucions regionals mostren tres estadis evolutius pel cinturó de plecs i encavalcaments de Kuqa: i) una extensió mesozoica menor; ii) una compressió primerenca (Cretaci superior fins Miocè inferior) amb taxes d’escurçament i sedimentació baixes; i iii) un estadi de compressió tardana (Pliocè superior-Pleistocè) caracteritzat per un creixement major i progressiu de les taxes d’escurçament i sedimentació. Per tal d’esclarir la influència de la taxa de sedimentació, els canvis laterals en la reologia dels décollements, i la interacció entre décollements en l’estil de deformació de l’avantpaís de cinturons de plecs i encavalcaments presentem un estudi experimental que inclou quatre models analògics 3D inspirats en el cinturó de plecs i encavalcaments de Kuqa. Els resultats experimentals mostren que augmentant la taxa de sedimentació es retarda el desenvolupament d’estructures contractives frontals desenganxades a la sal, afavorint la formació i reactivació d’encavalcaments i retro-encavalcaments a les zones internes. El nostre estudi revela que a mesura que la viscositat del décollement pre-cinemàtic augmenta la deformació es propaga lentament cap a l’avantpaís. Per altre banda, les estructures sub-sal poden: (i) determinar la extensió areal de la sal i per tant l’extensió del cinturó de plecs i encavalcaments desenganxats en ell i, (ii) retardar o inclús prevenir la propagació de la deformació sobre el nivell salí cap a l’avantpaís.